1,360 research outputs found
Challenges to the DGP Model from Horizon-Scale Growth and Geometry
We conduct a Markov Chain Monte Carlo study of the Dvali-Gabadadze-Porrati
(DGP) self-accelerating braneworld scenario given the cosmic microwave
background (CMB) anisotropy, supernovae and Hubble constant data by
implementing an effective dark energy prescription for modified gravity into a
standard Einstein-Boltzmann code. We find no way to alleviate the tension
between distance measures and horizon scale growth in this model. Growth
alterations due to perturbations propagating into the bulk appear as excess CMB
anisotropy at the lowest multipoles. In a flat cosmology, the maximum
likelihood DGP model is nominally a 5.3 sigma poorer fit than Lambda CDM.
Curvature can reduce the tension between distance measures but only at the
expense of exacerbating the problem with growth leading to a 4.8 sigma result
that is dominated by the low multipole CMB temperature spectrum. While changing
the initial conditions to reduce large scale power can flatten the temperature
spectrum, this also suppresses the large angle polarization spectrum in
violation of recent results from WMAP5. The failure of this model highlights
the power of combining growth and distance measures in cosmology as a test of
gravity on the largest scales.Comment: 12 pages, 7 figures, 4 tables, minor revisions reflect PRD published
versio
Weighing Neutrinos with Galaxy Cluster Surveys
Large future galaxy cluster surveys, combined with cosmic microwave
background observations, can achieve a high sensitivity to the masses of
cosmologically important neutrinos. We show that a weak lensing selected sample
of ~100,000 clusters could tighten the current upper bound on the sum of masses
of neutrino species by an order of magnitude, to a level of 0.03 eV. Since this
statistical sensitivity is below the best existing lower limit on the mass of
at least one neutrino species, a future detection is likely, provided that
systematic errors can be controlled to a similar level.Comment: 4 pages, 1 figure, version accepted for publication in PR
Cosmological Information in Weak Lensing Peaks
Recent studies have shown that the number counts of convergence peaks
N(kappa) in weak lensing (WL) maps, expected from large forthcoming surveys,
can be a useful probe of cosmology. We follow up on this finding, and use a
suite of WL convergence maps, obtained from ray-tracing N-body simulations, to
study (i) the physical origin of WL peaks with different heights, and (ii)
whether the peaks contain information beyond the convergence power spectrum
P_ell. In agreement with earlier work, we find that high peaks (with amplitudes
>~ 3.5 sigma, where sigma is the r.m.s. of the convergence kappa) are typically
dominated by a single massive halo. In contrast, medium-height peaks (~0.5-1.5
sigma) cannot be attributed to a single collapsed dark matter halo, and are
instead created by the projection of multiple (typically, 4-8) halos along the
line of sight, and by random galaxy shape noise. Nevertheless, these peaks
dominate the sensitivity to the cosmological parameters w, sigma_8, and
Omega_m. We find that the peak height distribution and its dependence on
cosmology differ significantly from predictions in a Gaussian random field. We
directly compute the marginalized errors on w, sigma_8, and Omega_m from the
N(kappa) + P_ell combination, including redshift tomography with source
galaxies at z_s=1 and z_s=2. We find that the N(kappa) + P_ell combination has
approximately twice the cosmological sensitivity compared to P_ell alone. These
results demonstrate that N(kappa) contains non-Gaussian information
complementary to the power spectrum.Comment: 24 pages, 12 figures, 14 tables. Accepted for publication in PRD
(version before proofs
Cosmology with Standard Sirens: the Importance of the Shape of the Lensing Magnification Distribution
The gravitational waves (GWs) emitted by inspiraling binary black holes,
expected to be detected by the Laser Interferometer Space Antenna (LISA), could
be used to determine the luminosity distance to these sources with the
unprecedented precision of <~ 1%. We study cosmological parameter constraints
from such standard sirens, in the presence of gravitational lensing by
large-scale structure. Lensing introduces magnification with a probability
distribution function (PDF) whose shape is highly skewed and depends on
cosmological parameters. We use Monte-Carlo simulations to generate mock
samples of standard sirens, including a small intrinsic scatter, as well as the
additional, larger scatter from lensing, in their inferred distances. We derive
constraints on cosmological parameters, by simultaneously fitting the mean and
the distribution of the residuals on the distance vs redshift (d_L - z) Hubble
diagram. We find that for standard sirens at redshift z ~ 1, the sensitivity to
a single cosmological parameter, such as the matter density Omega_m, or the
dark energy equation of state w, is ~ 50%-80% tighter when the skewed lensing
PDF is used, compared to the sensitivity derived from a Gaussian PDF with the
same variance. When these two parameters are constrained simultaneously, the
skewness yields a further enhanced improvement (by ~ 120%), owing to the
correlation between the parameters. The sensitivity to the amplitude of the
matter power spectrum, sigma_8 from the cosmological dependence of the PDF
alone, however, is ~ 20% worse than that from the Gaussian PDF. At higher
redshifts, the PDF resembles a Gaussian more closely, and the effects of the
skewness become less prominent. These results highlight the importance of
obtaining an accurate and reliable PDF of the lensing convergence, in order to
realize the full potential of standard sirens as cosmological probes.Comment: 16 pages, 9 tables, 12 figures, submitted to MNRA
Breaking Cosmological Degeneracies in Galaxy Cluster Surveys with a Physical Model of Cluster Structure
Forthcoming large galaxy cluster surveys will yield tight constraints on
cosmological models. It has been shown that in an idealized survey, containing
> 10,000 clusters, statistical errors on dark energy and other cosmological
parameters will be at the percent level. It has also been shown that through
"self-calibration", parameters describing the mass-observable relation and
cosmology can be simultaneously determined, though at a loss in accuracy by
about an order of magnitude. Here we examine the utility of an alternative
approach of self-calibration, in which a parametrized ab-initio physical model
is used to compute cluster structure and the resulting mass-observable
relations. As an example, we use a modified-entropy ("pre-heating") model of
the intracluster medium, with the history and magnitude of entropy injection as
unknown input parameters. Using a Fisher matrix approach, we evaluate the
expected simultaneous statistical errors on cosmological and cluster model
parameters. We study two types of surveys, in which a comparable number of
clusters are identified either through their X-ray emission or through their
integrated Sunyaev-Zel'dovich (SZ) effect. We find that compared to a
phenomenological parametrization of the mass-observable relation, using our
physical model yields significantly tighter constraints in both surveys, and
offers substantially improved synergy when the two surveys are combined. These
results suggest that parametrized physical models of cluster structure will be
useful when extracting cosmological constraints from SZ and X-ray cluster
surveys. (abridged)Comment: 22 pages, 8 figures, accepted to Ap
Constraining the Dark Energy Equation of State with Cosmic Voids
Our universe is observed to be accelerating due to the dominant dark energy
with negative pressure. The dark energy equation of state (w) holds a key to
understanding the ultimate fate of the universe. The cosmic voids behave like
bubbles in the universe so that their shapes must be quite sensitive to the
background cosmology. Assuming a flat universe and using the priors on the
matter density parameter (Omega_m) and the dimensionless Hubble parameter (h),
we demonstrate analytically that the ellipticity evolution of cosmic voids may
be a sensitive probe of the dark energy equation of state. We also discuss the
parameter degeneracy between w and Omega_m.Comment: ApJL in press, growth factor corrected, parameter degeneracy
calculate
Constraining Cosmology with High Convergence Regions in Weak Lensing Surveys
We propose to use a simple observable, the fractional area of "hot spots" in
weak gravitational lensing mass maps which are detected with high significance,
to determine background cosmological parameters. Because these high-convergence
regions are directly related to the physical nonlinear structures of the
universe, they derive cosmological information mainly from the nonlinear regime
of density fluctuations. We show that in combination with future cosmic
microwave background anisotropy measurements, this method can place constraints
on cosmological parameters that are comparable to those from the redshift
distribution of galaxy cluster abundances. The main advantage of the statistic
proposed in this paper is that projection effects, normally the main source of
uncertainty when determining the presence and the mass of a galaxy cluster,
here serve as a source of information.Comment: 14 pages, 4 figures, accepted for publication in Astrophysical
Journa
An Evolving Entropy Floor in the Intracluster Gas?
Non-gravitational processes, such as feedback from galaxies and their active
nuclei, are believed to have injected excess entropy into the intracluster gas,
and therefore to have modified the density profiles in galaxy clusters during
their formation. Here we study a simple model for this so-called preheating
scenario, and ask (i) whether it can simultaneously explain both global X-ray
scaling relations and number counts of galaxy clusters, and (ii) whether the
amount of entropy required evolves with redshift. We adopt a baseline entropy
profile that fits recent hydrodynamic simulations, modify the hydrostatic
equilibrium condition for the gas by including approx. 20% non-thermal pressure
support, and add an entropy floor K_0 that is allowed to vary with redshift. We
find that the observed luminosity-temperature (L-T) relations of low-redshift
(z=0.05) HIFLUGCS clusters and high-redshift (z=0.8) WARPS clusters are best
simultaneously reproduced with an evolving entropy floor of
K_0(z)=341(1+z)^{-0.83}h^{-1/3} keV cm^2. If we restrict our analysis to the
subset of bright (kT > 3 keV) clusters, we find that the evolving entropy floor
can mimic a self-similar evolution in the L-T scaling relation. This degeneracy
with self-similar evolution is, however, broken when (0.5 < kT < 3 keV)
clusters are also included. The approx. 60% entropy increase we find from z=0.8
to z=0.05 is roughly consistent with that expected if the heating is provided
by the evolving global quasar population. Using the cosmological parameters
from the WMAP 3-year data with sigma_8=0.76, our best-fit model underpredicts
the number counts of the X-ray galaxy clusters compared to those derived from
the 158 deg^2 ROSAT PSPC survey. Treating sigma_8 as a free parameter, we find
a best-fit value of sigma_8=0.80+/- 0.02.Comment: 14 emulateapj pages with 9 figures, submitted to Ap
Supermassive Black Holes: Connecting the Growth to the Cosmic Star Formation Rate
In this Letter, we present a model connecting the cosmic star formation rate
(CSFR) to the growth of supermassive black holes. Considering that the
evolution of the massive black hole is dominated by accretion (Soltan's
argument) and that the accretion process can be described by a probabilistic
function directly regulated by the CSFR, we obtain the evolution of the black
hole mass density. Then using the quasar luminosity function, we determine both
the functional form of the radiative efficiency and the evolution of the quasar
duty-cycle as functions of the redshift. We analyze four different CSFRs
showing that the quasar duty-cycle, , peaks at and so
within the window associated with the reionization of the Universe. In
particular, depending on the CSFR. The mean
radiative efficiency, , peaks at with
depending on the specific CSFR used. Our
results also show that is not necessary a supercritical Eddington accretComment: accepted for publication in MNRAS Letters (5 pages, 6 figures), Some
typos fixed; MNRAS Letters 17 Aug 201
Resonant Scattering and Ly-alpha Radiation Emergent from Neutral Hydrogen Halos
With a state-of-the-art numerical method for solving the
integral-differential equation of radiative transfer, we investigate the flux
of the Ly photon emergent from an optically thick halo
containing a central light source. Our focus is on the time-dependent effects
of the resonant scattering. We first show that the frequency distribution of
photons in the halo are quickly approaching to a locally thermalized state
around the resonant frequency, even when the mean intensity of the radiation is
highly time-dependent. Since initial conditions are forgotten during the
thermalization, some features of the flux, such as the two peak structure of
its profile, actually are independent of the intrinsic width and time behavior
of the central source, if the emergent photons are mainly from photons in the
thermalized state. In this case, the difference , where
are the frequencies of the two peaks of the flux, cannot be less
than times of Doppler broadening. We then study the radiative transfer in
the case where the light emitted from the central source is a flash. We
calculate the light curves of the flux from the halo. It shows that the flux is
still a flash. The time duration of the flash for the flux, however, is
independent of the original time duration of the light source but depends on
the optical depth of the halo. Therefore, the spatial transfer of resonant
photons is a diffusion process, even though it is not a purely Brownian
diffusion. This property enables an optically thick halo to trap and store
thermalized photons around for a long time after the cease of the
central source emission. The photons trapped in the halo can yield delayed
emission, of which the profile also shows typical two peak structure as that
from locally thermalized photons. Possible applications of these results are
addressed.Comment: 25 pages, 10 figures, accepted for publication in Ap
- …